This invention relates to improving performance of and controlling oxide of nitrogen (NOx) emissions from a motor-generator set comprising a turbocharged diesel engine used to drive an electrical power generating machine.
Turbocharged internal combustion engines may produce significant quantities of NOx emissions due to high combustion temperatures and incomplete burning of the petroleum-based fuel. These NOx emissions are harmful to the environment and to human health. Many localities have legislated limits on stationary motor-generator set NOx emissions, preventing installation of useful machines.
Engineers have developed a number of systems which inject water or water-containing solutions into the combustion chambers of an engine to reduce NOx emissions and/or improve engine performance. Water injection inhibits formation of high-temperature pollutants such as NOx because the combustion process takes place at lower temperatures and the water also displaces a small amount of the air previously available for formation of NOx. Water injection produces some additional advantages. The fluid absorbs heat within the combustion chamber and provides for an even burning rate to prevent, or at least minimize, detonation of the fuel charge in the combustion chamber. In addition, the fluid diminishes the accumulation of carbon deposits in the combustion chambers.
As an example of a prior-art water injection system, U.S. Pat. No. 4,424,676 to Meiners discloses a system which injects a water-alcohol-air mixture into the engine intake air stream through nozzles at the outlet side of a turbocharger compressor. This system responds to manifold pressure rather than to any direct engine load measurement and is primarily designed to improve engine performance rather than to reduce NOx emissions. U.S. Pat. No. 4,440,116 to Stevenson shows a similar system.
U.S. Pat. Nos. 4,401,059 and 4,300,483 to Goodman et al. shows another water and air injection system responding to pressure in the intake manifold and/or engine speed. This system provides a continuously varying fluid injection rate, and is designed to reduce NOx emissions in a non-turbocharged engine.
U.S. Pat. No. 4,311,118 to Slagle shows a water injection system adaptable to different diesel engines which provides a constant water injection flow rate above certain engine speed, throttle, and oil pressure settings to achieve known benefits. Other water injection systems that are even less similar to the present invention are shown in U.S. Pat. No. 3,834,359 (Ando) and U.S. Pat. No. 4,279,223 (Csonka).
None of these systems are optimal for use in a stationary turbodiesel motor-generator set because pollution control on a stationary turbodiesel motor-generator set presents different problems than pollution control on other diesel engines such as vehicle engines. A generator engine operates at constant speed, and the load varies with the amount of power being generated. Most other diesel engines are operated at speeds and loads which vary depending on many factors. The systems described above for water injection were generally designed for vehicle engine systems, and some include means for dynamically varying the rate of water injection. These systems are more expensive than necessary and in fact do not produce optimal results in turbodiesel power generation systems. In particular, these variable flow systems may at some times inject more water than necessary into the intake air stream, resulting in poor engine operation, poor fuel economy, and waste of expensive distilled water, or these systems may not activate at the proper time to maintain NOx emissions within required levels.
Further, the systems known in the art require complex, costly, and relatively unreliable sensors such as pressure sensors. Some systems actually use several sensors of different types, multiplying the number of potential breakdown points. Diesel motor-generator sets are normally used in continuous operation, and it is undesirable to shut the engine down to service the water injection system.
There is a need for a simple, inexpensive, and reliable system which will allow turbodiesel power generation units to meet applicable emissions requirements.